Generally, control devices for temperature conditioning apparatus are made with a high input impedance to reduce the current flow and thus reduce the heat generated in the control device to maintain its operating temperature within certain required limits. When such control devices are used with thermostatic switching devices or thermostats having circuits requiring a small "trickle" current for maintaining the circuit active during the off or open cycle of the thermostat, a high impedance control device may interpret a small "trickle" current as a call for operation of the control device. The "trickle" current might be used to charge batteries during the off cycle of the thermostat or maintain certain resistance capacitance (RC) circuits energized for standby power in the case of a power failure.
The present invention is concerned with a control device having a positive temperature coefficient (PTC) impedance which changes impedance with temperature. During the off cycle of the thermostat, when only the "trickle" current passes through the control device, the PTC impedance is low and a fairly large trickle current can exist. Upon a call for heat by the thermostat to bypass the trickle current circuit and deliver a larger current to the control device, the temperature of the PTC impedance will increase and thus its impedance will increase to reduce the wattage dissipated in the control device limiting the temperature of the control device within predetermined required limits.